Access to the popular social networking service Twitter and email service Hotmail has been blocked across mainland China, two days before the 20th anniversary of a bloody crackdown on Tiananmen Square.

Indignant users filled chatrooms with protest, after access to Twitter was denied shortly after 5:00pm (7:00pm AEST) on Tuesday.

"The whole Twitter community in China has been exploding with it," said Beijing-based technology commentator Kaiser Kuo.

"It's just part of life here. If anything surprises me, it's that it took them so long."

Thursday is the 20th anniversary of June 4, 1989, when tanks rolled into Tiananmen Square before dawn to quell weeks of protest by students and workers.

China has never released a death toll from the crackdown on what it classes as a "counter-revolutionary" conspiracy.

Other internet users reported not being able to access Windows Live, a service offered by Microsoft which also owns Hotmail, and also Flickr, an online photo sharing service owned by Yahoo.

"This is so frustrating. Now I feel China is exactly the same as Iran," said a financial professional and avid Twitter user in Shanghai, referring to Iran's May ban of popular social networking site Facebook.

Users in Beijing reported accessing the service without difficulty earlier on Tuesday, and even successfully searching potentially sensitive words such as "Tiananmen".

While professional and urban Chinese often use foreign internet tools, including Twitter, Hotmail and Facebook, the vast majority of Chinese use similar domestic services that are carefully monitored for any sign of content deemed subversive.

Access to video-sharing site YouTube, owned by Google, was blocked in China in March, after overseas Tibetan groups posted graphic footage of China's crackdown on protests by Tibetans in 2008.

I get the impression that there has been a general push by many governments to lock-down the internet and control the flow of information. I'm afraid the practices of China and Iran are only going to spread to other countries. Just look at the proposed internet filter for Australia as an example. The US once pursued a policy of "containment" against the spread of communism. I'm afraid there is nothing of the sort for spread of automated censorship.

Maybe they should start commemorating Tienanmen on a different date...

I'm never surprised by the the lengths the Chinese government is willing to go to in order to stem any sort of outflow of real opinion from their people. Ah well, if this is/was like most of the bans, then it should be (have been) over soon.